利用openssl_random_pseudo_bytes和ba

    private function getRandomString($length = 42)
    {
        if (function_exists('openssl_random_pseudo_bytes')) {
            $bytes = openssl_random_pseudo_bytes($length * 2);

            if ($bytes === false)
                throw new RuntimeException('Unable to generate a random string');

            return substr(str_replace(['/', '+', '='], '', base64_encode($bytes)), 0, $length);
        }

        $pool = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

        return substr(str_shuffle(str_repeat($pool, 5)), 0, $length);
    }